Work on Lock21 began in July 1829 and was completed in October 1830 at a cost of $8,327.76 (). The lock was made from Seneca Creek Red
Sandstone boated down the Potomac River from the
Seneca Quarry. Construction of the lock house began in May1831, and was finished in August1832 at a cost of $765.00 (). On August7, 1830, an individual listed only as "Fuller" was recommended and approved as
lock keeper (a.k.a. locktender). His annual compensation was $50 () with the additional benefits of the use of the lock house and the right to use the canal company's land, which was typically used for farming. By June1832, a 22-mile (35 km) section of the canal was operating between Georgetown and
Seneca, which included Lock21. Some C&O Canal records remain, allowing some of the lock keepers to be identified. Mrs. Susan Cross was lock keeper in 1836 until females were banned effective May 1 of that year. Exceptions were made, and most female lock keepers were widows or relatives of the previous keeper. Another early keeper for Lock21 was Robert C. Fields, who is listed as lock keeper on July 1, 1839. Samuel M. Fisher replaced Fields on May 1, 1846, after an incident at the lock. Fisher was still listed as lock keeper at the end of 1850. Thomas Tarman is listed as Lock21 tender for 1865. A map of
Montgomery County, Maryland, confirms Tarman as the "L.K." (lock keeper) at a point on the canal southwest of Offutt's Crossroads. The name Offutt's Crossroads comes from Edward Offutt, who received a large land grant from
Lord Baltimore in 1714. In 1881, the community was renamed
Potomac because the
Post Office said too many communities had "Crossroads" in their name. Today, Lock21 (Swains Lock) has a Potomac address and is located in the
Travilah census-designated place. The flood of 1889 caused damages to the entire canal estimated to be $1 million (). In the case of the Lock21 lock house, the upstream end wall was swept away. Repairs to the house included an addition that had a main floor lower than the original portion. The house's chimney was enlarged, and another was built on the downstream side. This addition made the house one of the largest lock houses on the canal. John Sipes is listed as the Lock21 lock keeper in a 1903 newspaper article. He drowned later that year at a lock described as "Gibbs Lock", located "about three miles above the Great Falls".
Swain family Lock21, Swains Lock, is named after the Swain family, which has been associated with the C&O Canal since early in its existence. Earlier, the lock had been known as Oak Spring Lock. John T. Swain Sr. was involved in the construction of the C&O Canal and a boatman. Most of his children were born on canal boats. His four sons were all involved with the canal as boatmen or boat captains: John T. Swain Jr., Charles Henry "Hen" Swain, William F. "Bill" Swain, and Jesse A. Swain. John Swain Sr. had 15 canal boats that he sold because the canal company would no longer allow them on the canal. A partial list of canal employees shows a dozen workers named Swain, and many of them were boat captains and a few were lock tenders. At Lock21, Jesse and his family supplemented their income by farming on land near the lock house and by driving a wagon of children to school during the offseason. One of Jesse's sons, Otho Oliver Swain, was born on a boat, worked as a boatman, and is thought to be the author of a
folk song about the canal. Jesse Swain died in 1939, survived by six of his children. Jesse's son Robert Lee "Bob" Swain and his wife Virginia moved into the lock house after Jesse died, and ran the family business.{{cite news ==Today==